1. Field of the Invention
The present invention relates to an electron gun for a cathode-ray tube and, more particularly, to an electrode structure in which it is possible to enlarge the diameter of the opening of a principal lens.
2. Description of the Related Art
FIG. 2 is a diagrammatic cross-sectional view of a cathode-ray tube provided with an electron gun employing a conventional BPF (bi-potential-focusing) type of principal lens. In the figure, the inner surface of a face plate 2 which constitutes a part of a glass envelope 1 is covered with a phosphor screen 3, and a triode section which serves as electron-beam generating means is constituted by a cathode 4, a G1 electrode 5, and a G2 electrode 6. An electron beam 11 is generated in the triode section and immediately focused to form a cross-over. Immediately after the cross-over, the electron beam 11 diverges and is again focused by a principal lens which is constituted by a focusing electrode 7 and an accelerating electrode 8. The accelerating electrode 8 is electrically connected through a spring contact 9 to a conducting layer 10 which is deposited on the inner surface of a predetermined portion of the glass envelope 1, whereby an equipotential space which extends between the accelerating electrode 8 and the phosphor screen 3 is formed. The electron beam 11 which is focused by the principal lens passes through the equipotential space and forms a beam spot on the phosphor screen 3. A magnetic deflection yoke 12 is provided so as to allow scanning of the beam spot on the phosphor screen 3. The G1 electrode 5, the G2 electrode 6, the focusing electrode 7, and the accelerating electrode 8 are spaced apart at predetermined intervals in the axial direction and secured in a coaxial relationship by means of electrode supporting rods 13 and 13' made of an insulating material such as glass.
The BPF type principal lens is formed by applying a focusing voltage having, for example, a low potential of approximately 5-10 kV to the focusing electrode 7 and, at the same time, applying an accelerating voltage having, for example, a high potential of approximately 20-35 kV to the acelerating electrode 8.
One primary factor which seriously influences the resolution characteristics of a cathode-ray tube is the spherical aberration of the principal lens thereof. As is well known, the enlargement of the diameter of the opening of each electrode which constitutes a principal lens is effective in reducing the spherical aberration of the principal lens. However, the diameter of the electrode opening is limited by the inner diameter of the neck of the glass envelope 1 which accommodates the electron gun, and it is not preferable to enlarge the inner diameter of the neck for the purpose of enlarging the diameter of the electrode opening since this would inevitably involve an increase in the electric power required for deflection.
In addition, the diameter of the electrode opening is limited by the electrode supporting rods 13 and 13' and it is thus impossible to fully enlarge the diameter of the electrode opening to coincide with the inner diameter of the neck.
For example, in the case of a cathode-ray tube having a neck with an outer diameter of 29 mm, the inner diameter of the neck is approximately 24 mm. Accordingly, if the presence of the electrode supporting rods 13 and 13' and the wall thickness of each electrode is taken into account, the respective diameters of the openings of the focusing electrode 7 and the accelerating electrode 8 are limited to approximately 12-13 mm.
Japanese Patent Examined Publication No. 58-31696 discloses a method which enables the diameter of an electrode opening to be made greater than the size as limited by electrode supporting rods. The structure of a principal lens utilizing this method will be described below with reference to FIG. 3.
The diameter of the opening portion of an accelerating electrode 18 which opposes the phosphor screen 3 is made as large as possible without going beyond the limit at which the outer surface of the accelerating electrode 18 would come into contact with the inner surface of the neck of the glass envelope 1. A focusing electrode 17 is partially disposed in the accelerating electrode 18, and the diameter of the opening portion of the focusing electrode 17 is enlarged to a size which does not allow any deterioration in the high-voltage resistance characteristic of the focusing electrode 17 with respect to the accelerating electrode 18. The respective portions of the focusing electrode 17 and the accelerating electrode 18 which oppose a triode section are reduced in diameter, and the reduced-diameter portions of the electrodes 17 and 18 are secured to each other by means of electrode supporting rods 113 and 113'.
With the above-described structure, it is possible to fully enlarge the diameter of the opening portion of the accelerating electrode 18 to coincide with the inner diameter of the neck of the glass envelope 1 in the vicinity of the opening portion of the focusing electrode 17 which opposes the accelerating electrode 18 and in which an electron leans is formed. For example, in the case of a cathode-ray tube having a neck with an outer diameter of 29 mm, the diameter of the opening portion of the accelerating electrode 18 can be made as large as 21 mm, and hence the diameter of the opening portion of the focusing electrode 17 can be made as large as 16 mm.
Several problems remain to be solved, however, in the related art described above. For example, although the accelerating electrode 18 can be made as large as possible without going beyond the limit at which the accelerating electrode 18 would come into contact with the inner surface of the neck, the diameter of the opening of the focusing electrode 17 cannot be sufficiently enlarged since its opening portion is disposed in the accelerating electrode 18. However, enlargement of the diameter of the opening portion of the focusing electrode 17 would be more effective in improving the spherical aberration characteristics of the principal lens. Accordingly, insufficient enlargement of the diameter of the opening portion of the focusing electrode 17 is unfavorable in terms of the desire to reduce the spot diameter of an electron beam and to enhance resolution.